jmc

We have $36-4x^2=6^2-(2x{)}^2=(6-2x)(6+2x)$. We can factor a 2 out of each of $6-2x$ and $6+2x$ to give $2\cdot (3-x)\cdot 2\cdot (3+x)=\boxed{4(3-x)(3+x)}$. (We could also have factored out a 4 at the beginning: $36-4x^2=4(9-x^2)=4(3-x)(3+x)$.)